Recently, various secondary batteries have been proposed as the power source for portable devices and mobile devices or as the power source for electric vehicles or hybrid electric vehicles. When the secondary batteries are to used as the power source for electric vehicles or hybrid electric vehicles, high output power is demanded and hence a plurality of secondary batteries are electrically connected in series to be used as a battery pack (an assembled battery).
However, in a battery pack used as the power source for electric vehicles or hybrid electric vehicles, one or more of the secondary batteries constituting the battery pack may be broken or reach the end of its life earlier than other batteries due to environments of use, characteristic difference among secondary batteries of the battery pack, or troubles of component parts of each secondary battery. As a result, the intended performance of the battery pack may not be exhibited, which may cause the entire system failure. To solve such problems, a secondary battery broken earlier than other batteries must be replaced with a normal secondary battery. Various methods have been proposed for replacing the secondary batteries (see, for example, Patent Literature 1).    Patent Literature 1: JP2004-185915A
Patent Literature 1 discloses a method of replacing secondary batteries of a battery pack in which a plurality of secondary batteries are electrically connected in series or in parallel. According to this method, when a certain secondary battery targeted for exchange is to be replaced with a new secondary battery for exchange, a charge amount of the new secondary battery is controlled to be smaller than charge amounts of other batteries not to be replaced (normal secondary batteries of the battery pack). Specifically, the new secondary battery is charged in advance so that the charge amount thereof is smaller by 5% to 20% than the charge amounts of other normal batteries not to be replaced. It is described that when charging and discharging are repeated during use of the battery pack, consequently, the difference in charge amount (the charged electric quantity) between the new secondary battery and other existing batteries becomes smaller, and hence the charge amounts of secondary batteries of the battery pack become equal. It is also said that the performance of the battery pack can be exhibited to a maximum extent.